In many excitable cells, a train of action potentials is followed by an after hyperpolarization (AHP). The AHP lasts up to seconds and prevents further action potentials during this period. Therefore, the AHP is an important regulatory of cell excitability. In some tissues, the AHP is inhibited by several neurotransmitters via activation of protein kinase A and calcium/calmodulin-dependent kinase II. The channels which underly this current have now been cloned, and the aim of this proposal is to use the cloned channels to examine the molecular mechanisms underlying their modulation. The ability of protein kinase A and calcium/calmodulin- dependent kinase II to inhibit macroscopic currents and single channel activity and to stimulate phosphate incorporation in vitro will be examined. Following, site-directed mutagenesis of consensus phosphorylation sites will be employed to identify the site of channel modulation.